There are several hereditary neuropathies, associated with mutation of genes encoding either myelin or specific Schwann cell proteins. These genetic pathologies of Schwann cells include the Charcot-Marie-Tooth disease (CMT), hereditary neuropathy with liability to pressure palsies (HNPP), Roussy-Levy syndrome and congenital hypomyelinating neuropathy (CHN).
Charcot-Marie-Tooth disease, also known as peroneal muscular atrophy, covers several disorders characterized by progressive deterioration of peripheral innervation. At least some of the incidences of CMT disease are the autosomal-recessive demyelinating neuropathy, which is caused by transmission of mutated gene(s), encoding the so-called 'ganglioside-induced differentiation-associated protein 1' (or GDAP1), which is believed to be involved in permanent bridging of Schwann cells and axons. Other variants of CMT are associated with duplication of chromosome 17. The disease is characterized by rapidly developing demyelination and axonal degradation, which trigger paralysis and early death. Additionally, some versions of CMT result from mutations in connexin 32, the main gap junctional protein in the PNS. Roussy-Levy syndrome is a variant of CMT, which is manifested by tremor and weakness in upper limbs, sensory loss and ataxia; it may arise from the mutation of the gene encoding P0, which is involved in adhesion of compact myelin (see Chapter 8), or from chromosomal disorders, e.g. particle duplication of chromosome 17.
Hereditary neuropathy with liability to pressure palsies results from deletion of point mutation in the same chromosome 17, which affect the gene encoding PMP22, which stabilizes the myelin sheath. HNPP pathology stems from dysmyeli-nation, as numerous redundant layers, loops or folds of myelin (known as tomac-ulae) are produced. This altered myelin production eventually results in prominent axonal death. Finally, congenital hypomyelinating neuropathy can be caused by various mutations, which affect the production of myelin. In particular CHN can be associated with mutations in the gene encoding periaxin, which is important for Schwann cell-axonal interactions. Alternatively hypomyelination syndromes can result from mutations in dystrophin gene.
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